In the printing and publishing industry, the increasing modularity of manufacturing operations is enabling customization of products. At the same time, pressures to reduce inventories and to keep them fresh are driving a trend toward just-in-time production and stocking. Wherever the manufacturing can be decentralized and distributed geographically, just-in-time production is facilitated because producers are closer to consumers in space and time. There is an ecological dividend, as well, in reduced demands on the transportation system. Overall product cost may decrease with shipping expense. At the same time, however, the challenge of maintaining uniform quality across a network of production sites increases. Minimizing startup waste gains in importance as does compensating for uneven skill and experience of operators. Color is a key variable to control because it affects product appearance and perceived quality.
Today for example, a magazine with a national circulation of 5 million may be printed at 5 regional plants scattered across the nation. Distribution (transportation and postage) generally account for one third of the cost of the product while transit time has a significant impact on product "freshness," i.e., the timeliness of the information delivered.
Production is as centralized as it is partly in order to maintain reasonably homogeneous quality. Nevertheless, printed color varies within a press run and from site to site because there have been only limited means of coordinating control of product appearance among sites. The scope and significance of this problem is apparent when one considers how much commerce and economic activity are leveraged by advertising and that generally more than 60% of all printing is advertising-related. Analogous problems also arise in other media, particularly now that digital video images can be edited in real time and broadcast directly.
The preceding paragraphs have spoken about parallel mass-production at multiple sites. Publishing is also distributed in the sense that the sequential steps of preparation for volume production occur at distinct sites, as illustrated in FIG. 1. Oftentimes, the sites represent different business entities (for example, an advertising agency, a publisher, or an "engraver") which are geographically separated. Solid lines in FIG. 1 represent links connecting the sites in the production process. Overlaid in FIG. 1 are dotted boundaries indicating a cluster of pre-publishing facilities which handle sequential phases of the process under Product Prototype 1, and printing facilities which may be involved in parallel Volume Production 2.
Currently prevalent volume printing technologies such as offset lithography, employ a printing "plate" which bears fixed information and is the tool or die of volume production. The tool is mounted on a press and numerous copies of the printed product are stamped out. For technologies such as ink jet and electrophotography the information on the plate can be changed from one revolution of the press to the next. This technological development enables significant product customization and is compatible with just-in-time production scenarios. It also enables process control in which the electronic data flowing to the device are modified to adapt to changes in the marking engine. However, the consistency (or repeatability) of these processes makes them even more susceptible to regional variations in quality across the production sites than lithography and its relatives.
For all of the printing technologies mentioned, there is a common problem of uniform and accurate color reproduction. Analogous problems also exist in other media for distributing color graphic or image content, such as CDROM or the Internet. Consider an advertiser in New York, physically removed from the five production sites mentioned above, or the more numerous sites that may be involved in the future. There is a keen interest in having the product portrayed in as faithful an accord with the advertiser's artistic conceptions as possible, even when the ad is to appear in different publications printed on different substrates by different machinery or in the same publication disseminated through different media.
Today, the approval cycle, the means by which print buyer and printer reach contractual agreement about the acceptability of product, often proceeds as outlined in FIG. 2, in the publication segment of the industry. Phases or functions of production are enclosed in ellipses 1a, 1b and 1c and key products of theses functions are enclosed by rectangles 3, 5, 6, 7, 8 and 9. The dashed line between creation 1a and prepress 1b shows the blurring of those functions in the development of intermediate products, such as page constituents like lineart, images, text and comps. Prepress 1b on the way to film 5 may include rasterization, separation and screening 4. However, acceptance of computer-to-plate technology will blur the boundary between prepress 1b and production 1c.
The long, heavy boundary line between press-proofing in low volume reproduction 1c and high volume production 2 represent the distinctness of the two functions; the former is carried out by engravers or commercial printers. Note that volume production 2 may occur at multiple sites. Linkages in the approval process are shown by arcs 10a and 10b at the bottom of FIG. 2, where 10a is the traditional off-press proof and 10b is a press proof. The transactions in the approval process involve one or more generations of static proofs which are prepared with limited regard for the capabilities of the final, volume-production devices. In other words, there is no feedback from production to earlier functions. The process results in idle time for equipment and personnel and waste of consumables (paper, ink etc.) Furthermore, it usually does not give the print buyer any direct say about the appearance of the ultimate product unless the buyer travels to the printing plant, an expensive proposition.
The workflow for commercial printing is slightly different from that described above, since press-proofs are seldom used and the print buyer or his agent often go to the printer's for approval. However, the essential lack of feedback is also prevalent in the commercial environment as well.
It is clear that a common language of color could insure improved communication, control and quality throughout the sites of FIG. 1. The common language is a color space, typically based on the internationally accepted Standard Observer which quantifies color in terms of what normal humans see, rather than in terms of specific samples or instances of color produced by particular equipment. The Standard Observer is the basis of device-independent, colorimetric methods of image reproduction and is defined by the Commission Internationale de L'Eclairage in CIE Publication 15.2, 1986, Central Bureau of the CIE, Box 169, Vienna, Austria. Approximately uniform perceptual color spaces based upon the Standard Observer are also discussed in this publication.
Color Space is defined as a three-dimensional, numerical scheme in which each and every humanly perceivable color has a unique coordinate. For example, CIELAB is a color space defined by the CIE in 1976 to simulate various aspects of human visual performance. Color in the next section will refer to CIE color or what we see, while colorant will refer to particular physical agents, such as dyes, pigments, phosphors, and the like that are instrumental in producing sensations and perceptions of color in a human at rendering devices, such as presses and video screens.
Prior Art
An early machine for converting color image data to colorant specifications for a 3 or 4-channel reflection reproduction process was described by Hardy and Wurzburg (Color correction in color printing, J. Opt. Soc. Amer. 38: 300-307, 1948.) They described an electronic network provided with feedback to control convergence to the solution of an inverse model of colorant mixture and produce 4-colorant reproductions "indistinguishable" from 3-colorant reproductions made under like conditions. The set point for the control was the color of the original. This work serves as a starting point for many subsequent developments in the art particularly as regards device independent color reproduction technologies and "color separation," i.e., the preparation of printing plates for 3 or more colorants.
In U.S. Pat. No. 2,790,844, Neugebauer discloses a system to extend the Hardy-Wurzburg machine. It describes the capture and representation of color imagery in a colorimetric (or device independent) coordinate system. To enable an operator to judge the effect of color corrections while he is making these color corrections, the system provides for a soft proof realized by projecting video images onto the type of paper stock to be used in the final reproduction with careful regard to making the surround illumination and viewing conditions comparable to those prevailing when the final product is viewed. The objective of the soft proof was to simulate a hard copy proof or final print. This is in contrast to U.S. Pat. No. 4,500,919, issued to Schreiber, which discloses a system to match the hard copy to the monitor image.
Concerning models of color formation by combination of colorants, Pobboravsky (A proposed engineering approach to color reproduction, TAGA Proceedings, 1962, pp. 127-165) first demonstrated the use of regression techniques ("curve fitting") to define mathematical relationships between device independent color ("in the CIE sense") and amounts of colorant with accurate results. The mathematical relationships took the form of low order polynomials in several variables.
Schwartz et al. (Measurements of Gray Component Reduction in neutrals and saturated colors, TAGA Proceedings, 1985, pp. 16-27) described a strategy for inverting "forward" models (mathematical functions for converting colorant mixtures to color.) The algorithm was similar to Hardy and Wurzburg's but implemented with digital computers; it consists of iteratively computing (or measuring) the color of a mixture of colorants, comparing the color to that desired and modifying the colorants in directions dictated by the gradients of colorants with respect to color error until color error is satisfactorily small. Color error is computed in CIE uniform coordinates. The context of the work was an implementation of an aspect of the art known as Gray Component Replacement (GCR.)
Because normal human color perception is inherently 3-dimensional, the use of more than 3 colorants is likely to involve at least one colorant whose effects can be simulated by a mixture of two or more of the others ("primaries.") For example, various amounts of black ink can be matched by specific mixtures of the "primary" subtractive colorants cyan, magenta and yellow. The goal of Schwartz et al. was a method for finding colorimetrically equivalent ("indistinguishable" in Hardy and Wurzburg's words) 4-colorant solutions to the problem of printing a given color that used varying amounts of black. Additional colorants (more than 3) are used to expand the gamut; black enables achievement of densities in reflection reproduction processes that are not otherwise available. A gamut is the subset of human perceivable colors that may be outputted by a rendering device. However, increased reliance on black through GCR has other important dividends: a) there is an economic benefit to the printer and an environmental benefit at large in using less colored ink, b) use of more black affords better control of the process.
Boll reported work on separating color for more than four colorants (A color to colorant transformation for a seven ink process, SPIE Vol. 2170, pp. 108-118, 1994, The Society for Photo-Optical and Instrumentation Engineers, Bellingham, Wash.). He describes the "Supergamut" for all seven colorants as a union of subgamuts formed by combinations of subsets of 4-at-a-time of the colorants. Because of the manner in which his subsets are modelled, the method severely limits flexibility in performing GCR.
Descriptions of gamuts in colorimetric terms date at least to Neugebauer (The colorimetric effect of the selection of printing inks and photographic filters on the quality of multicolor reproductions, TAGA Proceedings, 1956, pp. 15-28.) The first descriptions in the coordinates of one of the CIE's uniform color spaces are due to Gordon et al. (On the rendition of unprintable colors, TAGA Proceedings, 1987, pp. 186-195.) who extended the work to the first analysis of explicit gamut operators--i.e., functions which map colors from an input gamut to correspondents in an output gamut.
A detailed review of requirements of and strategies for color systems calibration and control was published by Holub, et al. (Color systems calibration for Graphics Arts, Parts I and II, Input and output devices, J. Image. Technol., 14: 47-60, 1988.) These papers cover four areas: a) the application of color measurement instrumentation to the calibration of devices, b) requirements for colorimetrically accurate image capture (imaging colorimetry,) c) development of rendering transformations for 4-colorant devices and d) requirements for soft proofing.
Concerning the first area (a), U.S. Pat. No. 5,272,518, issued to Vincent, discloses a portable spectral colorimeter for performing system-wide calibrations. The main departure from the work of Holub et al., just cited, is in the specification of a relatively low cost design based on a linearly variable spectral filter interposed between the object of measurement and a linear sensor array. Vincent also mentions applicability to insuring consistent color across a network, but does not discuss how distributed calibration would be implemented. There is no provision for self-checking of calibration by Vincent's instrument nor provision for verification of calibration in its application.
U.S. Pat. No. 5,107,332, issued to Chan, and U.S. Pat. No. 5,185,673, issued to Sobol, disclose similar systems for performing closed-loop control of digital printers. Both Chan and Sobol share the following features: 1) They are oriented toward relatively low quality, "desktop" devices, such as ink jet printers. 2) An important component in each system is a scanner, in particular, a flat-bed image digitizer. 3) The scanner and printing assembly are used as part of a closed system of calibration. A standardized calibration form made by the printing system is scanned and "distortions" or deviations from the expected color values are used to generate correction coefficients used to improve renderings. Colorimetric calibration of the scanner or print path to a device independent criterion in support of sharing of color data or proofing on external devices was not an objective. 4) No requirements are placed upon the spectral sensitivities of the scanner's RGB channel sensitivities. This has ramifications for the viability of the method for sets of rendering colorants other than those used in the closed printing system, as explained below.
In Sobol, the color reproduction of the device is not modelled; rather the "distortions" are measured and used to drive compensatory changes in the actual image data, prior to rendering. In Chan, there appears to be a model of the device which is modified by feedback to control rendering. However, colorimetric calibration for the purposes of building gamut descriptions in support of proofing relationships among devices is not disclosed.
Pertaining to item (b) of the Holub, et al. paper in J. Imaging Technology and to the foregoing patents, two articles are significant: 1) Gordon and Holub (On the use of linear transformations for scanner calibration, Color Research and Application, 18: 218-219, 1993) and 2) Holub (Colorimetric aspects of image capture, IS&T's 48th Annual Conference Proceedings, The Society for Imaging Science and Technology, Arlington, Va, May, 1995, pp. 449-451.) Taken together, these articles demonstrate that, except when the spectral sensitivities of the sensor's channels are linear combinations of the spectral sensitivity functions of the three human receptors, the gamut of an artificial sensor will not be identical to that of a normal human. In other words, the artificial sensor will be unable to distinguish colors that a human can distinguish. Another consequence is that there is generally no exact or simple mathematical transformation for mapping from sensor responses to human responses, as there is when the linearity criterion set forth in this paragraph is satisfied by the artificial sensor.
To summarize the preceding paragraphs: The objective of measuring the colors of reproduction for the purpose of controlling them to a human perceptual criterion across a network of devices in which proofing and the negotiation of approval are goals is best served when the image sensors are linear in the manner noted above.
Results of a colorimetric calibration of several printing presses were reported by Holub and Kearsley (Color to colorant conversions in a colorimetric separation system, SPIE vol. 1184, Neugebauer Memorial Seminar on Color Reproduction, pp. 24-35, 1989.) The purpose of the procedure was to enable workers upstream in the production process in a particular plant to be able to view images on video display devices, which images appeared substantially as they would in production, consistent with the goals of Neugebauer in U.S. Pat. No. 2,790,844. Productivity was enhanced when design could be performed with awareness of the limitations of the production equipment. The problem was that the production equipment changed with time (even within a production cycle) so that static calibration proved inadequate.
In U.S. Pat. No. 5,182,721, Kipphan et al. disclose a system for taking printed sheets and scanning specialized "color bars" at the margin of the sheets with a spectral colorimeter. Readings in CIELAB are compared to aim values and the color errors so generated converted into corrections in ink density specifications. The correction signals are passed to the ink preset control panel and processed by the circuits which control the inking keys of the offset press. Operator override is possible and is necessary when the colorimeter goes out of calibration, since it is not capable of calibration self-check. Although the unit generates data useable for statistical process control, the operator must be pro-active in sampling the press run with sufficient regularity and awareness of printed sheet count in order to exploit the capability. The process is closed loop, but off-line and does not read image area of the printed page. Important information regarding color deviations within the image area of the press sheet is lost by focussing on the color bars.
On page 5 of a periodical "Komori World News" are capsule descriptions of the Print Density Control System, which resembles the subject of Kipphan et al. Also described is the Print Quality Assessment System, which poises cameras over the press. The latter is primarily oriented toward defect inspection and not toward on-line color monitoring and control.
Sodergard et al. and others (On-line control of the colour print quality guided by the digital page description, proceedings of the 22nd International Conference of Printing Research Institutes, Munich, Germany, 1993 and A system for inspecting colour printing quality, TAGA Proceedings, 1995) describe a system for grabbing frames from the image area on a moving web for the purposes of controlling color, controlling registration and detecting defects. The application is in newspaper publishing. Stroboscopic illumination is employed to "freeze" frames of small areas of the printed page which are imaged with a CCD camera. The drawback of the Sodergard et al. system is that color control lacks the necessary precision for high quality color reproduction.
Optical low pass filtering (descreening) technology relevant to the design of area sensors for imaging colorimetry is discussed in U.S. Pat. No. 4,987,496, issued to Greivenkamp, and Color dependent optical prefilter for the suppression of aliasing artifacts, Applied Optics, 29: 676-684, 1990.)
Paul Shnitser (Spectrally adaptive acousto-optic tunable filter for fast imaging colorimetry, Abstract of Successful Phase I Proposal to U.S. Dept. of Commerce Small Business Innovation Research Program, 1995) and Clifford Hoyt (Toward higher res, lower cost quality color and multispectral imaging, Advanced Imaging, April '95) have discussed the applicability of electronically tunable optical/spectral filters to colorimetric imaging.
In "Thin-film measurements using SpectraCube.RTM.," (Application Note for Thin Film Measurements, SD Spectral Diagnostics Inc., Agoura Hills, Calif. 91301-4526) Garini describes a spectral imaging system employing " . . . a proprietary optical method based on proven Fourier spectroscopy, which enables the measurement of the complete visible light spectrum at each pixel . . . "
The applicability of neural network (and other highly parallel and hybrid) technologies to the calibration and control of rendering devices has been considered by Holub ("The future of parallel, analog and neural computing architectures in the Graphic Arts," TAGA Proceedings, 1988, pp. 80-112) and U.S. Pat. No. 5,200,816, issued to Rose, concerning color conversion by neural nets.
A formalism from finite element analysis is described in Gallagher, "Finite element analysis: Fundamentals," 1975, Englewood Cliffs, N.J. Prentice Hall, pp. 229-240 for use in the rapid evaluation of color transformations by interpolation.
Area (d) of the earlier discussion of Holub et al.'s review referred to principles guiding the design and application of softproofing: methods of calibrating video displays, evaluation of and compensation for illumination and viewing conditions, representation of how imagery will look on client devices and psychophysical considerations of matching appearance across media.
In the article "A general teleproofing system," (TAGA Proceedings, 1991, The Technical Association of the Graphic Arts, Rochester, N.Y.) Sodergard et al. and others discuss a method for digitizing the analog image of an arbitrary monitor for transmission through an ISDN telecommunications link to a remote video display. The method involves the transmission of the actual image data, albeit at the relatively low resolution afforded by the frame buffers typical of most displays. This method lacks any provision for calibration or verification of the devices at either end of a link and also lacks the data structures needed to support remote proofing and negotiation of color approval.
In U.S. Pat. No. 5,231,481, Eouzan et al. disclose a system for controlling a projection video display based on cathode ray tube technology. A camera is used for capturing image area of a display. The procedures are suited to the environment in which the displays are manufactured and not to where they are used. Concepts of colorimetric calibration of the display and control of display output to a colorimetric criterion are not disclosed.
In U.S. Pat. No. 5,309,257, Bonino et al. disclose a method for harmonizing the output of color devices, primarily video display monitors. In a first step, measurements of the voltage in vs. luminance out relationship are made for each of the three display channels separately and then the V/L functions of all the devices are adjusted to have a commonly achievable maximum. This is assumed to insure that all devices are operating within the same gamut--an assumption which is only true if the chromaticities of the primaries in all devices are substantially the same. The single-channel luminance meter (a "photometer") described as part of the preferred embodiment does not permit verification of the assumption. Bonino et al. thus employs photometric characterization of devices and lacks a colorimetric characterization.
The Metric Color Tag (MCT) Specification (Rev 1.1d, 1993, Electronics for Imaging, Inc., San Mateo, Calif. is a definition of data required in data files to allow color management systems to apply accurate color transformations. The MCT thus does not provide a file format defining the full specification of color transformations in the context of distributed production and color-critical remote proofing.
In contrast to the MCT, the International Color Consortium (ICC) Profile Format is a file format, and is described in the paper, International Color Consortium Profile Format (version 3.01, May 8, 1995). A "profile" is a data table which is used for color conversion--the translation of color image data from one color or colorant coordinate system to another. The ICC Profile Format provides for embedding profiles with image data. This generates large data transfers over a network whenever profiles are updated. Further, the ICC Profile, Representation of devices in the ICC Profile Format is limited in supporting "scnr" (scanner), "mntr" (video display monitor) and "prtr" (printer) device types, and is not readily extendable to other types of devices.
Interactive remote viewing is described for "imagexpo" application software from Group Logic, Inc., in the article "Introducing imagexpo 1.2: Interactive remote viewing and annotation software for the graphic arts professional" and "Before your very eyes," (reprinted from Publishing & Production Executive, August '95), which acknowledges that extant tools do not enable remote handling of color-critical aspects of proofing.